The present invention relates to hydraulic systems for work vehicles such as backhoes.
Conventionally, work vehicles such as backhoes are furnished with a pair of left and right hydraulic travel motors, a plurality of hydraulic cylinders, and service ports that can supply pressurized oil to the actuator (service actuator) for externally detachable attachments, and also are furnished with a hydraulic system that can simultaneously operate these hydraulic actuators. The hydraulic system includes a tank for storing pressurized oil, a main hydraulic pump and a sub hydraulic pump that can be driven by the engine, and a plurality of supply circuits that connect the hydraulic devices, hydraulic pumps, and the tank. The supply circuits are switched in accordance with the working conditions in order to supply a suitable amount of pressurized oil to each hydraulic actuator.
In one such common hydraulic system, if the travel motors are not to be driven, then the pressurized oil from the main hydraulic pump is supplied to the other hydraulic actuators excluding the travel motors, and if the travel motors are to be driven, then the pressurized oil from the main hydraulic pump is supplied only to the travel motors and the sub hydraulic pump supplies pressurized to the other hydraulic actuators.
In recent years, however, attachments that are attached and detached from the outside and furnished with service actuators that require more pressurized oil than in the past have been developed, and along with this there have been proposals for a hydraulic system that allows the pressurized oil from the sub hydraulic pump to be combined with the pressurized oil that is supplied by the main hydraulic pump to the service actuators during times when the travel motors are not driven.
There have been proposals for such a hydraulic system in which it is possible to select whether or not to combine the pressurized oil from the sub hydraulic pump in accord with the amount of oil that is required to drive the service actuator of the attachment that has been attached to the work vehicle.
For example, JP H11-36378A proposes a hydraulic system for a work vehicle that is provided with a switching means for switching between combining and not combining the pressurized oil from the sub hydraulic pump with the pressurized oil that is supplied to the service actuator by the main hydraulic pump, and the switching means is provided with a switch valve for switching, under pilot pressure, between combining the pressurized oil from the sub hydraulic pump with the pressurized oil from the main hydraulic pump and draining the pressurized oil of the sub hydraulic pump into the tank, and a stopcock that connects and disconnects a pilot control circuit for supplying the pilot pressure to the switch valve.
With the above hydraulic system, however, the switching means switches between combining and not combining the pressurized oil that is supplied by the sub hydraulic pump with the pressurized oil that is supplied by the main hydraulic pump to the service actuator in order to adjust the amount of pressurized oil that is supplied to the service actuator. For this reason, it was difficult to adjust the pressurized oil amount to an ideal amount for any one of various types of attachments, and this led to cases where various types of attachments could not be operated appropriately.
Further, with the above hydraulic system, the pilot control state of the switch valve is switched by opening and closing the stopcock, and through pilot actuation of the switch valve in the pilot control state, the switch valve is switched between a state of merging pressurized oil from the sub hydraulic pump with the pressurized oil from the main hydraulic pump and a state of draining the pressurized oil of the sub hydraulic pump into the tank. Thus, not only was it not possible to switch between the state of merging pressurized oil from the sub hydraulic pump with the pressurized oil from the main hydraulic pump or the state of draining it into the tank by opening and closing the stopcock alone, but the structure of the device was needlessly complicated as well, and this led to problems such as a larger valve unit and an increase in cost.